Blue Sunset On The Red Planet

This a NASA/JPL Mars rover Opportunity photo of the sunset at the end of the rover's 20th martian day. The Opportunity turned the panorama camera on its mast toward the horizon and captured this dusty blue martian sunset. (AP Photo/NASA/JPL)

The Opportunity rover turned its panoramic gaze on the horizon and captured a dusty blue sunset.

The six-wheeled robot snapped photographs not for the sake of beauty, but because scientists want to learn about the properties of dust in the atmosphere.

"It's pretty cool," pancam lead scientist Jim Bell said Thursday at NASA's Jet Propulsion Library as he showed a movie made from images recorded about three minutes apart.

A bluish tint and rapid dimming of the sun at the horizon were caused by dust scattering the sunlight, just as smog might do on Earth.

"Those of you who live in Los Angeles are very familiar with this effect," Bell said. "What's happening is the sun is sort of setting into the murky, dusty atmosphere of Mars."

Opportunity found twice as much dust in the atmosphere as was seen by NASA's Pathfinder spacecraft at another location on Mars in 1997. Pathfinder and two Viking spacecraft that landed in the 1970s also took sunset pictures.

Opportunity also was ordered to reduce its daily communications and take more powered-down "naps" to conserve electricity. Less sunlight is reaching the solar panels as Mars heads into its winter season.

Next week, the rover team will attempt something that's never been done: photographing an eclipse from the surface of another planet, Bell said. A rover camera will attempt to record the passages of the Martian moons Phobos and Deimos across the face of the sun.

Meanwhile on the other side of the planet, the twin rover Spirit was told to attempt to photograph dust devils spinning across the Martian surface. The rover also will take a closer look in coming days at a rock dubbed "Humphrey."